U.S. patent number 4,146,845 [Application Number 05/881,306] was granted by the patent office on 1979-03-27 for audio amplifier output circuit.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Richard E. Lunquist.
United States Patent |
4,146,845 |
Lunquist |
March 27, 1979 |
Audio amplifier output circuit
Abstract
A PNP/NPN complementary pair of common emitter connected output
transistors are driven by separate differential amplifiers having
unity feedback and biasing to prevent simultaneous operation. Only
a single battery supply is required.
Inventors: |
Lunquist; Richard E. (Pompano
Beach, FL) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
25378211 |
Appl.
No.: |
05/881,306 |
Filed: |
February 27, 1978 |
Current U.S.
Class: |
330/255; 330/260;
330/261; 330/268; 381/121 |
Current CPC
Class: |
H03F
3/183 (20130101); H03F 3/3067 (20130101); H03F
3/30 (20130101) |
Current International
Class: |
H03F
3/181 (20060101); H03F 3/183 (20060101); H03F
3/30 (20060101); H03F 003/183 () |
Field of
Search: |
;330/252,255,257,260,261,262,263,268 ;179/1A |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Turner et al., "Amplifier With Current Output," IBM Technical
Disclosure Bulletin, vol. 13, No. 8, Jan. 1971, pp.
2314-2315..
|
Primary Examiner: Mullins; James B.
Attorney, Agent or Firm: Parker; Margaret Marsh Gillman;
James W.
Claims
What is claimed is:
1. An audio frequency circuit comprising in combination:
a source of input signal;
a single source of supply voltage;
a reference voltage terminal;
first and second power output stages comprising a complementary
pair of transistors and connected in series between the supply
voltage source and the reference terminal;
a first output terminal connected to the connection point of the
first and second output stages and a second output terminal
connected to the reference voltage terminal;
a first differential amplifier means coupled to control the first
power output stage and having a first portion for receiving an
input signal and a first biasing voltage, a second portion coupled
to receive a second biasing voltage from the output of the first
power output stage;
a second differential amplifier means coupled to control the second
power output stage and having a first portion for receiving an
input signal and a third biasing voltage, and a second portion
coupled to receive a fourth biasing voltage from the second power
output stage, the first and third biasing voltages having a fixed
offset so that the first differential amplifier does not provide an
output voltage when the second differential amplifier is providing
an output voltage;
comparator means for receiving an input signal; and
feedback means coupled to the first output terminal and to the
comparator means for altering the input signal to remove from the
output signal of the circuit any discontinuity caused by the offset
in the first and third biasing voltages.
2. An audio frequency circuit in accordance with claim 1 wherein
each of the first and second differential amplifiers comprises a
pair of transistors, one pair being complementary to the other
pair.
3. An audio frequency circuit in accordance with claim 1 wherein
the first and second power output stages are coupled to actuate a
loudspeaker.
4. An audio frequency circuit in accordance with claim 1 wherein
the first and third biasing voltages are supplied from first and
second ends of a biasing resistor means, said resistor means being
coupled to a DC supply source and said offset voltage being
developed across said resistor.
5. An audio frequency circuit in accordance with claim 1 wherein
the output of the comparator means is coupled to the first and
second differential amplifier means.
6. An audio frequency circuit in accordance with claim 5 and
further including current mirror means coupled to the comparator
means for cooperation therewith in altering the input signal.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the field of audio output driver
stages and more particularly to such circuits as utilized in
integrated circuits in miniaturized equipment.
As electronic equipment has become increasingly miniaturized, the
circuitry required has typically become based on monolithic or
integrated circuitry. Power supplies for such circuitry still
remain a size problem, however, and the interconnections between
circuitry and battery supplies is frequently a problem. Common
emitter connected circuits have been known in the prior art, but
have required a tapped or split battery supply. It is desirable to
achieve the high efficiency of the common emitter connection for an
audio driver stage without requiring a split battery supply.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an
audio circuit for driving the output stages of an audio amplifier
without requiring a split battery supply.
It is a more particular object to provide this circuit by utilizing
a high efficiency common emitter connection in the driver
stages.
The above objects and others are achieved in a circuit in
accordance with the present invention wherein two power output
stages are connected in series to provide a high level signal for
operating a loudspeaker. Each of these output stages is driven by a
separate differential amplifier circuit. The two differential
amplifiers are coupled to receive an audio input signal, but do not
have the same biasing voltage. A resistor is connected in the
differential amplifier bias supply line in such a position that one
output stage will be turned off before the second one is turned on.
If no signal is being received at the input of the circuit, neither
output stage will be on. Each differential amplifier receives a
feedback signal from the corresponding output stage, and thus
tracks the input while it is conducting.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram of the circuitry according to the
invention.
FIG. 2 is a schematic diagram of the circuitry of FIG. 1.
FIG. 3 is a timing chart for the circuitry of FIG. 1
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
In the block diagram of FIG. 1, the terminal 10 supplies an audio
signal to the circuitry. This audio signal could come from any one
of a number of sources, such as a broadcast signal which has been
detected and demodulated, and is now to be amplified to a level
sufficient to operate a loudspeaker. The particular application in
which the circuitry is most desirable is a battery operated
portable unit but the invention is not so limited. The signal from
the terminal 10 is coupled to a differential comparator 11 which,
in conjunction with a current mirror 12, supplies a signal to a
voltage amplifier 13. The amplifier 13 is coupled in series with
the battery supply 16, a resistor 17 and a current source 18. The
purpose of the resistor 17 will be discussed in detail with respect
to FIG. 2. One end of the resistor 17 is coupled to a differential
amplifier 21 which in turn is coupled to a current source 22. The
other end of the resistor 17 is coupled to another differential
amplifier 24 which is coupled to a current source 25. The
differential amplifier 21 output is coupled to an output stage 26,
and feedback signals are coupled back to the differential amplifier
21 by a feedback loop 27. The differential amplifier 24 output is
coupled to an output stage 28, from which a feedback loop 29
couples back to the differential amplifier 24. The output stages 26
and 28 are coupled to operate a loudspeaker 31. A portion of the
input signal for the loudspeaker is coupled back to the
differential comparator 11 by way of another feedback loop 33.
The operation of the block diagram of FIG. 1 will be further
explained with respect to the schematic diagram of FIG. 2 and the
timing chart of FIG. 3. The audio signals from the terminal 10 are
coupled to a transistor Q1 of the differential comparator 11 and
the output signals fed back from the output stages 26 and 28 are
coupled to transistor Q2. As is known, such a feedback loop as the
loop 33 acts to remove distortion introduced within the circuitry
and, in this case, particularly the cross-over distortion as will
be explained hereinbelow. The current mirror 12 is formed of the
transistor Q3 and a diode-connected transistor Q4. The signal from
the collector of Q1 is coupled to a transistor Q5 which is one
embodiment of the voltage amplifier 13. This signal contains the
input signal plus a compensating signal to eliminate or attenuate
the cross-over distortion which may be introduced within the
circuitry.
The output of the voltage amplifier transistor Q5 is coupled to one
portion of the differential amplifier 21 which portion comprises a
transistor Q6. Also coupled to transistor Q6 is a biasing voltage
from the lower voltage side of the resistor 17. The second portion
of the differential amplifier 21 comprises a transistor Q7 and is
coupled to receive a feedback signal from the output stage 26. When
there is no signal input, the biasing voltage for the transistor Q7
is approximately B+/2. As the input voltage for Q6 begins to rise
(curve I, FIG. 3), the voltage at the collector of a transistor Q8
and hence the voltage on the base of Q7 will also rise and the
voltages at the bases of Q6 and Q7 will track, i.e., the transistor
Q6 supplies just enough current to the base of Q8 to enable the
voltage at the collector of Q8 to track the input signal at the
base of Q6. The remainder of the current from the current source 22
is diverted to ground through Q7. As the input for Q6 returns
toward zero, the output of the output stage 26 (curve D) will also
return. The output stage 26 may be embodied by a transistor Q8 as
shown in FIG. 2. The differential amplifier 24 is made up of
transistors Q9 and Q10, which will be PNP transistors if the
transistors Q6 and Q7 are NPN type and, with the biasing connection
as shown in FIG. 2, there will be a very brief interval between the
end of conduction in differential amplifier 21 and the beginning of
conduction in differential amplifier 24. The differential biasing
of resistor 17 keeps Q6 and Q9 from being on at the same time. Once
amplifier 24 is on, it behaves just as did amplifier 21, and the
output of Q10 tracks the input to Q9, providing the signals as
shown in curve II of FIG. 3. These signals are amplified in the
current amplifier Q11, and are coupled to the output signals from
Q8 to operate the loudspeaker 31.
FIG. 3, III illustrates the principle of operation of the circuitry
as the outputs would appear if the feedback loop 33 were not
included. Curve A is the signal on the base of transistor Q9 and
curve C is the signal on the base of Q6. Curve B would be the
combined signal outputs of Q7 and Q10. During the period "D", both
Q6 and Q9 are biased into nonconduction. During the period "E",
both B and C begin to rise and Q6 begins to conduct, with Q7
tracking. At the end of period "E", Q6 is turned off, but Q9 does
not begin to conduct until the beginning of period "F". Thus,
without the feedback loop 33 the composite signal B would have the
cross-over period indicated at "G". The feedback loop 33 from the
output stages Q8 and Q11, however, causes a compensating effect to
be introduced at the differential comparator 11, and the signal
coupled to the voltage amplifier 13 is adjusted to compensate for
the cross-over distortion term in curve D, and to produce an output
approaching or becoming the waveform seen in curve IV.
Since the differential amplifiers are capable of driving Q8 and Q11
into saturation, essentially the total swing of the power supply
can be used in the output.
By means of the present invention, an audio circuit has been
provided wherein the high efficiency of common emitter connected
circuitry is retained while a single, untapped power supply is
required. It will be apparent that other modifications and
variations of the invention are possible, and it is contemplated to
include all those which fall within the spirit and scope of the
appended claims.
* * * * *